Preferably, the office chair is equipped with a so-called synchronizing mechanism. The inclination of the backrest of such chairs can usually be adjusted counter a restoring force. In the case of comfort chairs it is provided that the restoring force can be set, in order to adapt it to the needs of users of different weights.
In this connection different mechanisms and methods are possible for setting purposes. For example, the prestress of a spring which exerts the restoring force can be set manually through an actuating element, such as a handwheel. However, to be able to adjust the spring prestress, a very high force is required, so that an expensive transmission is generally required, which also has the consequence that a relatively high number of revolutions are to be carried out to achieve a perceptible adjustment.
As an alternative, it can be provided to configure a whole spring assembly arrangement or, in general, the arrangement of the spring element to be pivotable, so that the articulation points of the spring element in the force parallelogram are changed.
Moreover, known setting possibilities often have the problem that the ratio of the initial force exerted by the spring element (in the rest position) to the maximum force (in the inclined position) often behaves unfavorably in the case of a performed weight setting. In other words, this means that the restoring force exerted by the spring element via the inclination adjustment of the chair backrest is perceived differently by a light person and a heavy person, for example in such a way that, in the case of a light person an initially perceived soft setting which is first of all perceived as pleasant is perceived more and more sluggish as the inclination increases, and vice versa in the case of a heavy person. Therefore, there is the problem of the correct setting beyond the inclination adjustment of the backrest.
Moreover, adjusting mechanisms are known from the prior art which, in order to set the restoring force, provide a change in an active lever length between a rotational axis of the backrest support and an action point of the spring element. Thus, for example, in the mechanism which is known from the international patent disclosure WO 2006/114250, a roller is adjusted with the aid of a setting lever, which roller is first guided along a surface on the backrest support and second along a surface of a pivoting lever, the roller and the pivoting lever moderating between the action point of the spring element and the backrest support in order to set different active lever lengths.
An adjusting mechanism can be gathered from published European patent application EP 1 258 212 A2, in which adjusting mechanism the action point of a spring element on the backrest support can be adjusted in order to change an active lever length between the action point and the pivot pin. In a similar way, EP 1 258 211 A2 describes a bearing block which can be displaced along a sliding guide in order to set different active lever lengths.
An adjusting mechanism similar to the present invention can be gathered from the international patent application WO 2011/141107 A1. The adjusting mechanism described therein comprises a setting mechanism which is configured in the manner of a pair of scissors for weight setting, thus for setting the restoring force in a weight-dependent manner. The pair of scissors comprises two scissor arms which are rotatably connected about a scissor pin. The first scissor arm is fixed by means of its second end to a backrest support and the second scissor arm is articulated with its second end to an articulation element in the form of a plate.
A spring element acts on this articulation element in order to exert the restoring force. The weight setting occurs by pivoting the scissors so that a spacing between the scissor pin and a support pin about which the backrest support is pivotable, is varied. Hereby the active lever length is varied in order to set the weight. By the special mechanism provided with the scissors and the articulation element, it is ensured that the spring element is arranged fixedly within a support.